Images from a flat-panel X-ray detector (FD) are square or rectangular. Images acquired from such an X-ray detector will normally be displayed in the same form on a display unit, e.g. a monitor. If it is necessary to rotate an acquired image, corners of the rectangular image will turn outside of the displayed area and are no longer visible, especially in a zoom mode.
Images acquired from a conventional X-ray image intensifier (II) are circularly shaped and thus are easily to rotate. It is not favourable to use circular images, because a circular image contains much less information than a square or rectangular image with the same overall dimensions.